High voltage potential transformer



March 9, 1965 u. A. PEURON HIGH VOLTAGE POTENTIAL TRANSFORMER 2Sheets-Sheet 1 Filed Oqt. 26, 1962 Fig. l

INVENTOR Unto A. Peuron ga f/W ATTORNEY March 9, 1965 u. A. PEURON3,173,115

HIGH VOLTAGE POTENTIAL TRANSFORMER Filed Oct. 26, 1962 2 Sheets-Sheet 2Fig. 2

United States Patent 0 3,173,115 HIGH VOLTAGE POTENTEAL TRANSFORMER UntoA. Peuron, Churchill Borough, Pa., assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed Oct.26, 1962, Ser. No. 233,302 6 Claims. (Cl. 336-84) This invention relatesin general to electrical inductive apparatus, such as transformers, andmore particularly to winding and insulation structures for suchapparatus.

The physical size of high voltage electrical inductive apparatus, suchas instrument transformers, is determined largely by insulationrequirements rather than the voltampere rating of the apparatus.Considerable size reduction in instrument transformers immersed in adielectric fluid, such as oil, has been accomplished by using a solidinsulation comprising fibrous insulating material, such as crepe paper.The barrier action of the fibrous insulating material allows asubstantial reduction in the major insulation. Further advantages areobtained when high voltage inductive apparatus, such as potentialtransformers, utilize the solid insulation and are constructed with asingle pyramidal type high voltage coil. Surge voltage distribution in apyramidal type primary coil with continuously narrowing width of windinglayers is excellent. The initial and final voltage distributions acrossa pyramidal type coil nearly coincide, indicating an absence of harmfuloscillations and making it possible to reduce insulating materialnecessary in other types of coil designs to meet special surge voltagecharacteristics.

However, even with the above reductions in major insulations, stillfurther reductions of major insulation could be made if the potentialstress surrounding the high voltage coil could be reduced. Still furtherreductions in major insulation could be achieved if the initial andfinal voltage distributions for the transformer as a whole wouldcoincide. Also, a large portion of the weight and cost in a high voltageinductive apparatus is in the weather casing or bushing for the primarylead. Condenser type bushings produce linear voltage distribution alongtheir outer surfaces, but for reasons of excessive length and high cost,it is highly advantageous to use simple bulk type bushings and utilizeother means for obtaining longitudinal stress control along the highvoltage bushing. Some prior art designs use a potential grading systemsimilar to that of a condenser bushing, except the step-locatedconcentric shields are incorporated in the primary lead insulationrather than in the bushing itself. This method, however, hasdisadvantages in that considerable skill and accuracy is required on thepart of the manufacturing personnel who insulate the leads. In addition,the primary lead in sulation has to be made relatively thick and with alarge plurality of shields in order to reduce the voltage steps and edgeeffect on shield ends. Too great a voltage between adjacent shieldsresults in corona on the sharp edges of the shields. it is, therefore,desirable to provide an improved winding and insulation arrangement thatwill provide favorable initial and final voltage distribution and stresscontrol along the high voltage bushing, as well as radial stress controlon coil corners, without the disadvantages of the hereinbefore mentionedsystems of voltage distribution. It is also desirable that the improvedwinding and insulation arrangement incorporate pyramidal type primarycoils and bulk type bushings, thus resulting in electrical inductiveapparatus having not only reduced insulation thickness but reducedcritical voltage gradients as well.

Accordingly, it is the general object of this invention to provide a newand improved winding and insulation structure for electrical inductiveapparatus, such as transformers.

Another object is to provide a new and improved shielding arrangementfor the high voltage winding of a transformer.

A more specific object of this invention is to provide a new andimproved high voltage instrument transformer, such as a potentialtransformer, which has substantially linear voltage distribution alongthe high voltage bushing.

A still further object of this invention is to provide a new andimproved high voltage potential transformer that has reduced potentialgradient adjacent the high voltage coil and static shield.

A further object of this invention is to provide a new and improved highvoltage potential transformer that has an initial and final voltagedistribution that are substantially the same.

Briefly, the present invention accomplishes the above cited objects byproviding a properly designed and located intermediate shield thatinterposes and divides the pyramidal type primary coil of the inductiveapparatus into two sections. More specifically, the intermediate shieldis connected to a portion of the high voltage coil, fixing the potentialof the intermediate shield at some point between the primary voltageapplied to the transformer and ground or zero potential. Theintermediate shield is interjacent the inner and outer shields andlocated relative to the inner and outer shields in such a manner thatthe intershield capacitances provide surge and final voltagedistributions that are substantially the same. Further, the intermediateshield considerably reduces critical gradients in the inductiveapparatus and provides favorable voltage distribution along the highvoltage bushing and radially to the high voltage coil.

Further objects and advantages of this invention will become apparent asthe following description proceeds and features of novelty whichcharacterize the invention will be pointed out in particuarity in theclaims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIGURE 1 is a front elevational view, partly in section, of atransformer constructed in accordance with the teachings of thisinvention;

FIG. 2 is a side elevational view, partly in section, of the transformershown in FIG. 1;

FIG. 3 is a front elevational View partly in section of a static shieldthat may be used in the transformer shown in FIGS. 1 and 2;

FIG. 4 is a diagrammatic representation of the shielding arrangementutilized in the transformer shown in FIGS. 1 and 2; and

FIG. 5 is a diagrammatic representation of the effective capacitiverelationships which exist in the shielding arrangement illustrated inFIG. 4.

Referring now to the drawings and FIGS. 1 and 2 in particular, there isillustrated a high voltage potential transformer 10 constructed inaccordance with the teachings of this invention. The transformer It)includes a coil and core assembly which comprises the primary or highvoltage winding 12 and the secondary or low voltage winding 14, alldisposed in an inductive relationship with the magnetic core 16, whichmay be of any conventional type. The high voltage winding is dividedinto sections 12A and 12B by intermediate shield 20, which will bedescribed in greater detail hereinafter. Lead 22 from high voltage coil12 extends radially outward and upward and is attached by soldering, orother suitable means, to terminal 21 on metallic cap 24. Primaryterminal 26, which is also suitably attached to metallic cap 24, may beconnected to a high voltage circuit whose potential is to be measured.The high and low voltage windings 12 and 14, along with magnetic core 16are suitu ably disposed within a casing or tank 28. The lead 22 from thehigh voltage winding 12 is enclosed in a lead tube 36 and lead tube isin turn enclosed in a suitable hollow cylindrical or tubular bushing orweather housing 32 whose central opening is tapered in cross-sectionalarea along its vertical axis. The casing 28 and bushing 32 are filledwith a suitable fluid dielectric, such as an insulating oil. Metalliccap 24 is a conventional expansion cap suitably disposed at the top ofbushing 32 to allow for expansion and contraction of the dielectricfluid provided inside casing 28 and bushing 32.

The bushing 32 is supported by and secured to the cover 34 of casing 28in any suitable manner, such as by flange member 36 and flange member38, which may be formed integrally with the top 34 of casing 28, asshown in FIGS. 1 and 2. The flange member 36 may be secured to theflange 38 and top 34 by any suitable means, such as bolts or a weldedconnection, similar to the construction disclosed in greater detail inU.S. Patent 2,508,- 184 which is assigned to the same assignee as thepresent application. Terminals 39 are low voltage terminals connected tothe secondary winding 14.

More specifically, the high voltage winding 12 is pyramidal in shape orhas continuously narrowing width of winding layers and is inductivelydisposed on a portion of magnetic core 16. The pyramidal type coil hasvery favorable voltage distribution characteristics and is less costlyto construct, insulate, and shield than the multicoil designs sometimesused. The low voltage winding 14 is disposed inside the high voltagewinding 12 and is sub stantially concentric with it. In order toinsulate the high voltage winding 12 from the low voltage winding 14 andfrom the grounded portions of the transformer Ill, solid insulation 4this disopsed to substantially surround the high voltage winding 12 and toprovide the necessary insulation between the high voltage Winding 12 andthe low voltage winding 14-. The solid insulation 40 may be any suitableinsulating material, such as crepe paper, in sheet or tape form, whichis either taped, wrapped or folded around the high voltage winding 12and between the high voltage winding 12 and the low voltage winding 14.

In order to provide a duct or channel through which fluid dielectric mayflow vertically either in an upward or downward direction for reasonswhich will be explained hereinafter, the tubular or hollow cylindricalinsulating member 30, formed of a phenolic laminate or other suitableinsulating material, is disposed in this instance around lead 22 betweenthe high voltage winding and the upper portion of said lead, insidebushing 32. The lower end of insulating member 30 is suitably attachedto hollow static ring 423, which will be described in detailhereinafter.

In order to additionally electrically insulate the high voltage lead 22from the grounded portions of the transformer 19, solid insulation 14%)is disposed to substantially surround the major vertical portion of saidlead. The solid insulation 140 may be similar to the solid insulation 4tdisposed around the high voltage winding 12, and may be crepe paper inflexible sheet form which is taped, wrapped, or folded around the lead22. The thickness of the solid insulation Mil is generally tapered froma maximum value at the lower portion of the lead 22 to a minimum valuenear the upper portion of said lead.

The entire solid insulation structure may be oil impregnated, using highvacuum techniques. The absence of oil ducts in the solid insulationsubstantially eliminates or reduces the possibility of voids or pocketsin the solid insulation surrounding the high voltage winding 12 and lead22, which would otherwise be filled with a dielectric fluid having alower specific inductive capacity than the associated solid insulation.The presence of such voids or pockets in areas of high dielectric stressis particularly undesirable at higher operating potentials, and mayeventually lead to an insulation failure or 4t breakdown in the solidinsulation surrounding the high voltage winding and its leads. This isdue to the fact that voltage is distributed between two media in amanner inversely proportional to the capacitance of the two materials.Since the capacitance of the materials is directly proportional to theirdielectric constants, the voltage distribution is greatly affected bythe dielectric constants of the two materials. Therefore, an oil pocket,with a dielectric constant of approximately 2 would be subjected to morethan its normal share of voltage where the dielectric constant of thesurrounding oil impregnated crepe paper insulation is 4. In addition,the breakdown strength of oil is lower than that of the solidinsulation.

In order to reduce the concentration of dielectric stress in the solidinsulation 10 surrounding the high voltage winding 12, particularlyadjacent to the outer ends or corners of said high voltage winding, andin the solid insulation surrounding the lead 22, the inner shieldingmember 42 having upper and lower portions 42A and 428, respectively, isdisposed to substantially surround the high voltage winding 12 and themajor portion of lead 22. The inner shielding member 42 alsosubstantially eliminates any potential stress which the fluid dielectricmight be subjected to in the channel or duct formed by the tubularinsulating member 39 surrounding the lead 22, as previously described.The lower portion 42B of the inner shielding member 42, sometimes calleda static ring, has several basic functions. t serves as a shield withrounded edges, it has an opening to the tubular insulating member 3t)and acts as an inner coo-ling duct, and it protects the high voltagecoil 12 mechanically and forms a rigid base to which the insulatingmember 3% can be attached.

FIG. 3 shows the static ring portion of the inner shield 4-2 in greaterdetail. The static ring 4233 is comprised of an outer tubular member 50,formed of pressboard, laminated plastic, or other suitable insulatingmaterial, having an opening 52 over which the insulating member 36 issuitably disposed. If pressboard is used, the meeting ends may be foldedto form a base to which the insulating member 36 may be attached bymeans of flexible insulating material in the form of tape.

An inner tubular member 54 is disposed concentrically Within the outertubular member 55) and may be constructed or" the same material as saidouter member. The inner member 54, however, has no opening or foldedends.

The inner and outer members 54 and 50, respectively, are concentricallylocated and held in place by spacing members 56 and 58, which may becorrugated pressboard strips fastened to said inner and outer members bycementing, or other suitable means. Spacing members 6t and 52 aresimilar to spacing members 56 and 58, except members 69 and 62 are shortpieces or strips so that dielectric fluid may flow freely along thelength of the static shield 42B.

Rounding means 64 and 66, located on both ends of the tubular assembly,partially between the inner and outer tubular members 54 and 5trespectively, may be formed of materials such as rope or metal tubing.If metal tubing is used it may function as the sensing por tion of acapillary thermometer mounted on top of bushing 32, thus allowing thehigh voltage coil temperature to be monitored visually.

The static shield 42B is completed by disposing a shielding memberaround the entire static ring assembly. More specifically, a conductingor semiconducting tape is so disposed as to cover the entire assembly,inside as well as out. In particular, the shielding member 7 0 coversthe rounded ends of the static ring 423 formed by members 64 and 66. Theshielding member portion 76b of static ring 42B is formed by tightlywinding a flexible conducting material having a layer of electricallyinsulating material secured thereto, such as; crepe paper backedmetallicfoil in the form of tape, around and through the static shield 42B. Thecircumferential ends of the lower portion 428 of the inner shield 42 arepreferably overlapped, but with a gap which is filled with solidinsulation 40 to prevent a short circuited turn around the magnetic core16.

Referring again to FIGS. 1 and 2, the upper portion 42A of inner shield42 may be formed by tightly winding a flexible conducting materialhaving a layer of insulation secured thereto, such as crepe paper backedmetallic foil, in the form of a sheet or tape, substantially around thetubular insulating member 31 prior to the assembly of the solidinsulation 141). The upper and lower portions 42A and 4213,respectively, of the inner shielding member 42 are electricallyconnected to one another to form a continuous conducting path throughoutthe shielding member 42 which will limit the potential differencebetween the different portions thereof to a negligible value so thatthere is no interruption in the shielding effect between the upper andlower portions of the shielding member 42.

The upper end of the inner shielding member 42 is electrically connectedto the terminal 21 by the flexible conducting lead 80 in order that theshielding member 42 provides a substantially equipotential surfacearound the high voltage winding 12 and high voltage lead 22, which is atsubstantially the same potential as the outermost layer of the highvoltage winding 12, to thereby sub stantially eliminate any potentialstress to which any fluid dielectric inside said shielding member issubjected. It should be noted that the fluid dielectric uses the upperand lower portions of the inner shield 42 as ducts or channels. Fluidflow is accomplished by the counterfiow method, wherein heated fluidrises up through the insulating member 31) and cooler fluid flows downsaid insulating member.

It is to be understood that although a separate lead 22 is shownconnected from the high volt-age coil 12 to terminal 21, the highvoltage coil 12 could just as effectively be connected to the metallicportion of the inner shield at the lower end of insulating member 34).Then, the only connection to terminal 21 would be a flexible lead fillfrom the metallic shield at the upper end of insulating member 30 toterminal 21. The shield around insulating member 30 may effectively beutilized as the high voltage lead because of the small currentsinvolved.

In order to reduce the concentration of dielectric stress in the solidinsulation 140 which surrounds lead 22 adjacent to the grounded portionsof the casing 28 through which said lead passes, and to substantiallyeliminate any potential stress to which the fluid dielectric issubjected inside the casing 28 and in the lower portion of the centralopening of the bushings 32, the outer shielding member 90 is disposed tosubstantially surround the high voltage winding 12 and its associatedsolid insulation 4%, as well as the lower portion of lead 22 and thesolid insulation 14% which is disposed around said leads.

The outer shielding member 90 is formed by winding around and throughthe central opening of the high voltage winding 12 a flexible conductingmaterial having a layer of insulation secured thereto for mechanicalpurposes, after the solid insulation 40 has been assembled around thehigh voltage winding 12 and by winding the same type of flexible sheetmaterial snugly and tightly around the outer surface of a solidinsulation 140 after the latter insulation has been assembled around thelead 22. The upper portion of outer shield 90 is embedded in the solidinsulation 140 which surrounds the lead 22 and the upper end of shield9th is folded back on itself, as indicated at 92, to form a stresscollar and reduce the concentration of dielectric stress in the adjacentsolid insulation 14% and fluid dielectric.

The outer shield 90 may be formed of materials such as aluminum foilbacked crepe paper or carbon backed crepe paper. When carbon backedcrepe paper is used, its higher resistivity makes it possible to allowthe shield to form a complete circuit around the core leg that carriesthe high and low voltage coils 12 and 14. When metal backed crepe paperis used, it is necessary to include a gap in the shielding, in order toprevent a short circuited turn around the magnetic core 16. Carbonbacked crepe paper has other advantages over metal backed crepe paper,in that wrinkles and sharp edges are not as detrimetal when using carbonbacked paper as opposed to metallic type shields. Also, the higherresistance of carbon has a potential grading effect because thedisplacement current due to stress concentration causes a voltage dropalong the semiconducting layer. This phenomenon is well known andutilized in condenser bushings and windings of high voltage generators.It is also known that due to so called surface activity, an insulationwith a semiconducting electrode, such as carbon, in the highest stressedboundary has higher dielectric strength than a system with metallicelectrodes only.

The outer shielding member forms a continuous conducting surface orelectrode having a cylindrical configuration around the high voltagewinding 12 and a generally hollow cylindrical shape around the lowerportion of lead 22. The outer shielding member 90 is maintained atground or zero potential by electrically connecting said shieldingmember by a flexible conducting lead as indicated at 94 in FIG. 1 to thecasing 28, or any other grounded portion of the transformer 10.

The outer shilding member 90 is disposed in substantially concentricrelation with the inner shielding member 42, and similar to the lattershielding member, the outer shielding member 90 forms a continuous,substantially equipotential surface around the outer surface of thesolid insulation 40 and the outer surface of the lower portion of thesolid insulation 140. The flexible shielding material from which theouter shielding member 98 is formed permits said shielding member toclosely follow the contour or outer surface of the solid insulation 40and prevents the occurrence or voids or pockets which would otherwise befilled with the associated fluid dielec trio and which would be subjectto possible insulation failure or breakdown.

In order to reduce the radial stresses on the static shield 42B and thehigh voltage coil 12, and produce a favorable voltage distributionlongitudinally along the bushing 32, an intermediate shielding member20, having upper and lower portions 20A and 2013, respectively, isdisposed to substantially surround the high voltage winding 12 and thelower portion of the lead 22. In addition to surrounding the highvoltage coil 12, the intermediate shield 24) interposes the high voltagecoil 12, dividing said high voltage coil into two portions or sections,12A and 1213. It is to be noted that the outer surface of the lowerportion 208 of intermediate shield 20 is embedded in the solidinsulation 40. The inner surface of said lower portion of intermediateshielding member 26 interposes and separates the high voltage winding 12into inner and outer portions designed as 12B and 12A, respectively. Theupper portion 20A of intermediate shielding member 20 is embedded in thesolid insulation which surrounds lead 22. The upper and lower portions20A and 21B of the intermediate shielding member 20 may be formed in amanner similar to the outer shielding member 90, as previouslydescribed, by winding a flexible conducting or semiconducting materialhaving a layer of insulation secured thereto, such as crepe paper withmetallic foil or carbon attached to it, in the form of a tape or sheet,around and innterposing the high voltage winding 12, after approximatelyhalf the solid insulation 40 has been assembled around the high voltagewinding and around the lower portion of the lead 22 after approximatelyhalf the solid insulation 140 has been assembled around said lead. Theintermediate shielding member 2013 includes a gap if a metallic shieldis used in order to prevent a short circuited turn around the magneticcore 16. As previously explained, no gap is necessary if asemiconducting shield, such as carbon, is used. The upper end of theupper portion A of the intermediate shielding member 29 is folded backon itself as indicated at 96 to form a stress collar and therebyincrease the effective radius of curvature of the upper end of saidshielding member and reduce the concentration of dielectric stress inthe adjacent solid insulation 14%) and fluid dielectric below the coronalevel.

The upper and lower portions and 26B, respectively, of the intermediateshielding member 20 are electrically connected with one another orformer from the same flexible conducting material to provide acontinuous, substantially equipotential surface around the high voltagewinding 12 and the lower portion of lead 22. Similar to the outershielding member 2 0, the intermediate shielding member 2f forms agenerally cylindrical electrode surface around the high voltage winding12 and also a generally cylindrical electrode surface around the lowerportion of lead 22 in substantially concentric or parallel relation withthe inner shielding member 42 and the outer shielding member 99. Unlikethe outer shielding member 9%, however, the intermediate shieldingmember 20 does not completely encircle the high voltage coil through thecentral portion of the coil opening. The intermediate shielding memberinterposes the high v oltage coil 12 at some intermediate point,dividing said high voltage coil into portions 12A and 1213. Thepotential of the intermediate shielding member 2t) is fixed to asuitable value by connecting said intermediate shielding member to thatpoint in the high voltage coil 12 having the desired inductively fixedalternating potential. The potential of the intermediate shield is fixedto a value between the potential applied to the transformer primaryterminal 26 and ground or zero potential, with the value usually beingbetween 34) and 70 percent of the applied potential. Locating theintermediate shield 20 in this manner, interposing and dividing the highvoltage coil 12 into two portions 12A and 12B, reduces appreciably themaximum voltage gradients on the static shield 42B by moving theintermediate shielding member 20 radially away from the location wherethe corresponding equipotential surface would be located in the absenceof said intermediate shield. By reducing the critical voltage gradientson the static shield 423, the intermediate shielding member 28 makespossible a marked reduction in major insulation and improves the coronacharacteristics of the transformer ill. Also, the reduction ininsulation makes heat transfer less of a problem and more efficientcooling is obtained, even when using a smaller volume of fiuiddielectric. A further advantage of the intermediate shielding member 20,as will be explained in greater detail hereinafter, is the morefavorable voltage distribution obtained longitudinally along the bushing32. This more linear voltage distribution along the bushing 32 alsocontributes to a reduction in the amount of fluid dielectric and majorinsulation required.

While the easiest manner of fixing the voltage of the intermediateshield 20 to the desired value is to actually make a physical connectionfrom the appropriate portion of the high voltage coil 12 to theintermediate shielding member Zil, it will be appreciated that thedesired potential at the intermediate shielding member 20 could also beobtained without any physical connections. The desired potential at theintermediate shielding member 20 could be obtained by proper arrangementof the shielding members 2 6, 42 and lit) with respect to each other andwith respect to the grounded portions of transformer ill.

The intermediate snielding member 2% not only reduces critical highstress areas on the static shield 42B, but also allows the capacitanceof the transformer to be adjusted to a value that substantiallyeliminates harmful transient oscillations that occur upon applying ahigh voltage to a transformer. Once current is flowing through atransformer, the voltage distribution is determined inductively or bymagnetic coupling and may be designed to be substantially uniform. Theinitial voltage distribution, upon application of the high voltage tothe transformer, may be entirely different from the final distribution,however, because initial voltage distribution is determined entirelyelectrostatically, or by the capacitances of the coils and turns to eachother and to the various shields. The greater the deviation of theinitial voltage distribution from the final voltage distribution, thegreater the harmful transient voltage oscillations produced as thevoltage distribution changes from capactive voltage distribution toinductive voltage distribution Therefore, it is essential that thetransformer be designed to provide substantially similar initial andfinal voltage distribution curves. The intermediate shield makes itpossible to obtain an initial voltage distribution that is substantiallysimilar to the final voltage distribution. For example, if theintermediate shield 29 is designed for fifty percent potential, then inaddition to the proper connection to the corresponding point in thecoil, the intermediate shielding member must be placed so that the totalcapacitance between the inner shielding member 42 and the intermediateshielding member 20 equals the capacitance between the intermediateshielding member 2Q and the outer shielding member .90 and othergrounded portions of the transformer it For potentials other than 50% atthe intermediate shield, it is only necessary to design the shields sothat the total capacitances between shields are inversely proportionalto the potential dill-erences under normal operating conditions.

More specifically, FIG. 4 illustrating diagrammatically the shieldingmembers 29, 42 and and the associated effective capacitances andinductances which exist between the different portions thereof as wellas the effective capacitances which exist between the shielding members2% and 42 and the grounded portions of the transformer 10. The effectivecapacitance between the lower portion 423 of shielding member 42 and thelower portion of the intermediate shielding member 268 is indicated at CThe effective capacitance which exists between the lower portion 233 ofintermediate shielding member 20 and the lower portion of outershielding member 90 is indicated at C The effective capacitance betweenthe upper portion 42A of the inner shielding member 42 and the upperportion 26A of intermediate shielding member 20 is indicated at C Theeffective capacitance between the upper portion 20A of intermediateshielding member 20 and the portion of outer shielding member 9% isindicated at C The effective capacitance between the upper portion 42Aof inner shielding member 42 and other grounded portions of transformer10 is indicated at G The effective capacitance which exists between theupper portion 20A of intermediate shielding member 2%) and othergrounded portions of the transformer 10 is indicated at C The lattercapacitive relationships which exist between the shielding members 2%,42 and 9t) and between certain of said shielding members and thegrounded portions of transformer 10 are indicated by the equivalentschematic diagram shown in PEG. 5. It is well known that if a pluralityof capacitors are connected in series and a potential applied to theseries circuit, the potential will divide or distribute itself acrossthe respective capacitors inversely in accordance with the capacitancevalue of said capacitors. By providing an intermediate shielding member2% having its respective portions disposed at an intermediate radialpoint in the solid insulation which surrounds the lead 22, and in thesolid insulation ill disposed around the high voltage winding 12 andinterleaving the high voltage winding 12 and by varying the effectivecapacitances between various portions of the shielding members 2t), 4-2and 9d, the potential at the radial point in the solid insulation atwhich the intermediate shielding member 2GB is disposed can be increasedto a value above that which would otherwise exist in the absence of theintermediate shielding member Ztl. Therefore, the maximum potentialgradient which exists at the outer corners of the high voltage winding12 adjacent to the inner shielding member or static shield 423 in thesolid insulation can be reduced, permitting an important reduction inthe amount of thickness of the solid insulation th required around thehigh voltage winding 12 and between the high voltage winding 12 and thelow voltage winding 314.

The intermediate shielding member 24 also produces substantial reductionin the insulation 14% surrounding lead 22 and the length and size ofbushing 32 by enabling the proper capacitances to be established betweenshielding members and between shielding members and the groundedportions of transformer 10, and thus obtaining a favorable longitudinalvoltage distribution along bushing 32.

Referring to FIG. and assuming C :C +C and C C -l-C the voltagedistribution is determined by the relationship E /E C /C 1f the designis based on equal voltages on both sides of the intermediate shieldingmember 26, C would equal C Otherwise, different capacitive relationshipswould exist. To obtain the desired capacitance relationships, it may benecessary to increase the total capacitance between the inner shieldingmember 42 and the intermediate shielding member 20. The totalcapacitance between the inner shielding member 42 and the intermediateshielding member 2% may be increased by moving the upper portion 20A ofthe intermediate shielding member 29 radially inward towards the upperportion 42A of inner shield 42, or by axially extending the length ofthe intermediate shielding member 2i) in an upward direction. Inaddition to designing the capacitive relationships between shieldingmembers and between shielding members and ground to providesubstantially linear voltage distribution along the bushing 32, saidcapacitive relationships are designed so that under impulse conditionsthe favorable voltage distribution is not disturbed. Flux mappingtechniques may be used to determine the elfective capacitance of thewindings upon initial application of the high voltage to thetransformer.

In summary, the effect of the intermediate shielding member in a windingand insulation structure as disclosed is threefold. First, theintermediate shielding member 2% reduces the maximum potential gradientwhich would otherwise exist at the surface of the inner shielding member4213 at the outer corners of the high voltage winding 12 and increasesthe potential gradient in the portions of the solid insulation 4% whichare radially removed from the outer corners of the high voltage winding12. It is important to note that it is possible to reduce the maximumpotential gradient in the manner disclosed by moving the upper portionZtiA of the intermediate shielding member 2d radiaily inward toward theupper portion 42A- of the inner shielding member 42, since the eifectiveradius of curvature of the inner shielding member 42, which is atsubstantially the same potential as the high voltage winding 19;, ismuch greater than the effective radius of curvature at the outer cornersof the high voltage winding =12 at the surface of the lower portion 423of the inner shielding member 42, although it is possible toalternatively extend the axial dimension of the upper portion ZGA of theintermediate shielding member 20 in an upward direction to obtain thedesired increase in total capacitance between the inner shielding member29 and the outer shielding member 90 in a particular application and toobtain the desired potential relationship between the respectiveshielding members. The intermediate shielding member 2h thus permits asubstantial reduction in the amount of thickness of solid insulationrequired around the high voltage winding 12 and substantially improvesthe space factor of the transformer 1h, since the required size of themagnetic core is correspondingly reduced as well as the physical weightand size of the overall transformer ltl. Second, further reduction ininsulation and high voltage bushing weight and size is attributed to theintermediate shielding member 20 in that proper placement of saidintermediate shielding member more uniformly distributes the voltagelongitudinally along said bushing. This eliminates the requirement forcondenser type bushings or step-concentric shields as hereinbeforedescribed. Third, the requirement for excessive insulation to preventinsulation failure from surges and voltage oscillations due to initialapplication of the voltage to the transformer is eliminated. Theintermediate shielding member 20 makes it possible to obtain capacitiverelationships that produce an initial voltage distribution substantiallysimilar to the final voltage distribution. Thus, there are substantiallyno harmful voltage oscillations as the voltage changes from. capacitiveor initial to inductive or final distribution.

It is to be understood that in certain applications, additionalintermediate shielding members may be employed to improve the potentialgradient in certain portions of the solid insulation and provide a morelinear voltage distribution along the high voltage bushing in atransformer of the type disclosed. Also, the shielding members asdisclosed may be formed from semiconducting or conducting materials ofother types, such as semiconducting rubber, flexible insulating materialwith a metallic coating sprayed therein, or Coronox tape. Further, it isto be understood that the outer shielding member 96) may be omitted incertain applications, with the metallic casing 28, which is normally atground po tential, functioning as an effective outer shield.

It will therefore be apparent that there has been disclosed a new andimproved winding, insulation, and shielding structure for electricalinductive apparatus that reduces the potential gradient next to the highvoltage coil and on the static shield adjacent the high voltage coil,provides a more favorable voltage distribution along the high voltagebushing, and provides an inductive device wherein the initial and finalvoltage distribution curves substantially coincide. The new and improvedwinding, insulation and shielding structure results in in severaladvantages. For example, the amount and radial thickness of the solidinsulation required in a transformer of the type described isconsiderably reduced, thus reducing the physical size and weight of thetransformer. Also, the high voltage bushing may be considerably smallerand lighter, further adding to the weight and size reduction of theoverall transformer. In addition, the reduction in maximum potentialgnadient results in reduced possibility of insulation failures due tobreakdown of the fluid dielectric. Finally, the reduction in solidinsulation reduces the effect of said solid insulation as a thermalbarrier and reduces the requirement for special cooling arrangements andreduces the amount of fluid dielectric required.

Since numerous changes may be made in the above described apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawings shall be interpreted as illustrative, and not in a limitingsense.

I claim as my invention:

1. A potential tranformer comprising a pyramidal type high voltagewinding, a lead electrically connected to and extending outwardly fromsaid high voltage winding, a first shielding member formed fromconducting material and connected to said lead disposed to substantiallysurround said high voltage winding and said lead, solid insulationsubstantially surrounding said high voltage winding, lead, and firstshielding member, a second shielding member formed from conductingmaterial and connected to ground potential disposed to substantiallysurround in a close fitting manner at least the major portion of saidsolid insulation, and an intermediate shielding member having lower andupper portions formed from conducting material disposed to surround saidhigh voltage winding and at least a portion of said lead, respectively,said lower portion of said intermediate shielding member being embeddedin said solid insulation substantially halfway between said first andsecond shielding members and interposing and dividing said high voltageWinding into two sections, said intermediate shielding member beingelectrically connected to said high voltage winding at a pointintermediate the start and finish of said high voltage winding, with thepoint on said high voltage winding being selected to obtain a potentialwhich provides the maximum reduction in potential gradient at said firstshielding member, said upper portion of said intermediate shieldingmember being positioned closer to said first shielding member than tosaid second shielding member to produce a capacitance relationshipbetween said shielding members that would force said intermediateshielding member to assume the same potential that said intermediateshielding member has assumed due to its being connected to the selectedpoint of the high voltage winding.

2. A potential transformer comprising a pyramidal type high voltagewinding, a lead electrically connected to and extending outwardly fromsaid high voltage winding, a first shielding member formed of first andsecond electrically connected sections and connected to said lead, saidfirst section of said first shielding member substantially surroundingsaid lead and said second section of said first shielding membersubstantially surrounding said high voltage winding, solid insulationsubstantially surrounding said high voltage winding, lead, and firstshielding member, a second shielding member formed from conductingmaterial and connected to ground potential and disposed to substantiallysurround in a close fitting manner at least the major portion of saidsolid insulation, and an intermediate shielding member having lower andupper portions formed from conducting material disposed to surround saidhigh voltage winding and at least a portion of said lead, respectively,said lower portion of said intermediate shielding member being embeddedin said solid insulation substantially halfway between said first andsecond shielding members and interposing and dividing said high voltagewinding into sections, said intermediate shielding member beingelectrically connected to the high voltage winding at a pointintermediate its start and finish, the potential at said point being ofa magnitude to increase the potential of the intermediate shieldingmember to a magnitude higher than would exist at the same location insaid solid insulation in the absence of said intermediate shieldingmember, to substantially reduce the potential gradient at said innershielding member, said upper portion of said intermediate shieldingmember being positioned closer to said first shielding member than tosaid second shielding member to produce a capacitance relationshipbetween said shielding members that causes the initial and final voltagedistribution across said transformer to be substantially the same.

3. A potential transformer comprising a high voltage winding, a leadelectrically connected to and extending outwardly from said high voltagewinding, a first shielding member formed from semiconducting materialand connected to said lead disposed to substantially surround said highvoltage winding and said lead, solid insulation substantiallysurrounding said high voltage winding, lead, and first shielding member,a second shielding member formed from semiconducting material andconnected to 1 ground potential disposed to substantially surround in aclose fitting manner at least the major portion of said solidinsulation, and an intermediate shielding member having lower and upperportions formed from a semiconducting material disposed to surround saidhigh voltage winding and at least a portion of said lead, respectively,said lower portion of said intermediate shielding member being embeddedin said solid insulation substantially halfway between said first andsecond shielding members and interposing and dividing said high voltagewinding into two sections, said intermediate shielding memberelectrically connected to said high voltage winding at a pointintermediate the start and finish of said high voltage winding, with thepotential of said point being selected to provide the maximum reductionin potential gradient adjacent said first shielding member, said upperportion of said intermediate shielding member heing positioned closer tosaid first shielding member than to said second shielding member toprovide a capacitance relationship that provides the same potential atthe intermediate shielding member that said intermediate shieldingmember has assumed due to its being connected to said high voltage coil,to substantially reduce transient voltages as the voltage distributionacross said transformer changes from a capacitance distribution to aninductive distribution.

4. A transformer comprising a pyramidal type high voltage winding, 21primary voltage terminal, a first shielding member formed of first andsecond electrically connected sections, said high. voltage windingelectrically connected to said first section of said first shieldingmember and said first section electrically connected to said primaryvoltage terminal, said second section of said first shielding membersubstantially surrounding said high voltage winding, solid insulationsubstantially surrounding said high voltage winding and first shieldingmember, a second shielding member electrically connected to groundpotential disposed to substantially surround in a close fitting mannerat least the major portion of said solid insulation, and an intermediateshielding member having lower and upper portions disposed to surroundsaid high voltage winding and first shielding member, the lower portionof said intermediate shielding member being embedded in said solidinsulation substantially halfway between said first and second shieldingmembers and interposing and dividing said high voltage winding intosections, said intermediate shielding member being electricallyconnected to said high voltage winding at a point between the start andfinish of said high voltage winding, with the potential at said pointbeing of a magnitude to increase the potential of the intermediateshielding member to a magnitude higher than would exist at the samelocation in said solid insulation in the absence of said intermediateshielding member, to substantially reduce the potential gradient at saidhigh voltage winding and at said first shielding member, said upperportion of said intermediate shielding member being positioned closer tosaid first shielding member than to said second shielding member toproduce a capacitance relationship between said shielding members thatforces the initial and final voltage distribution across the transformerto be substantially the same.

5. A transformer comprising a metallic casing, a high voltage windingdisposed in said casing, at least one lead electrically connected tosaid winding and extending outwardly therefrom, solid insu ationsubstantially surrounding said winding and at least a portion of saidlead, a first shielding member formed of conducting material anddisposed between said solid insulation and said winding and between saidsolid insulation and lead to substantially surround said winding andleads, said first shielding member being electrically connected to saidlead, and a second shielding member having lower and upper electricallyconnected portions formed from conducting material and substantiallysurrounding said winding and said lead, respectively, said lower portionof said second shielding member interposing and dividing the highvoltage winding into two sections and positioned substantially halfwaybetween said first shielding member and the outer surface of said solidinsulation, said second shielding member being electrically connected tosaid high voltage winding at a predetermined point, with the potentialat the point increasing the potential of the second shielding member toa magnitude higher than would exist at the same location in said solidinsulation in the absence of said second shielding member, tosubstantially reduce the potential gradient at the first shieldingmember, said upper portion of said secend shielding member beingpositioned closer to said first shielding member than to the outersurface of said solid insulation to produce a capacitance relationshipbetween said first and second shielding members and between said secondshielding member and said casing that p row'des an initial voltagedistribution across said transformer that is substantially the same asthe final voltage distribution.

6. An instrument transformer comprising a metallic casing, a highvoltage winding disposed within said casing, 21 lead electricallyconnected to said winding and extending outwardly therefrom, solidinsulation substantially surrounding said winding and at least a portionof said lead, a first shielding member electrically connected to saidlead comprising lower and upper sections disposed between said solidinsulation and said winding and between said solid insulation and saidlead respectively, the lower section of said first shielding memberbeing hollow and cylindrical in shape and having conducting material onits surface, the upper section of said first shielding member being atubular insulating member braving a conducting material on its externalsurfaces, the upper and lower sections of said first shielding memberbeing mechanically and electrically connected, and a second shieldingmember having lower and upper portions formed from conducting materialand substantially surrounding said winding and said lead, respectively,said lower portion of said second shielding member being disposedsubstantially halfway between said first shielding member and the outersurface of said solid insulation and interposing and dividing said highvoltage Winding into two portions, said second shielding member beingelectrically connected to said high voltage winding at a predeterminedpoint, the point being selected to provide a potential that increase thepotential of said intermediate shielding member to a magnitude greaterthan said intermediate shielding member would assume by its positionalone to reduce the maximum potential gradient adjacent to the surfaceof said first shielding member and to provide a more linear voltagedistribution along said high voltage lead, said second shielding memberbeing positioned relatively closer to said first shielding member thanto the outer surface of said solid insulation to produce a capacitancerelationship that provides initial and final voltage distributionsacross said transformer that are substantially the same.

References Cited in the file of this patent UNITED STATES PATENTS2,331,106 Oamilli Oct. 5, 1943 2,359,544 Carnilli Oct. 3, 1944 3,028,568Camilli Apr. 3, 1962 3,086,184 Nichols Apr. 16, 1963 FOREIGN PATENTS64,792 Denmark Sept. 2, 1946 208,456 Germany Sept. 15, 1959 440,018Italy Oct. 4, 1948 1,239,860 France July 18, 1960

1. A POTENTIAL TRANSFORMER COMPRISING A PYRAMIDAL TYPE HIGH VOLTAGEWINDING, A LEAD ELECTRICALLY CONNECTED TO AND EXTENDING OUTWARDLY FROMSAID HIGH VOLTAGE WINDING A FIRST SHIELDING MEMBER FORMED FROMCONDUCTING MATERIAL AND CONNECTED TO SAID LEAD DISPOSED TO SUBSTANTIALLYSURROUND SAID HIGH VOLTAGE WINDING AND SAID LEAD, SOLID INSULATIONSUBSTANTIALLY SURROUNDING SAID HIGH VOLTAGE WINDING, LEAD, AND FIRSTSHIELDING MEMBER, A SECOND SHIELDING MEMBER FORMED FROM CONDUCTINGMATERIAL AND CONNECTED TO GROUND POTENTIAL DISPOSED TO SUBSTANTIALLYSURROUD IN A CLOSE FITTING MANNER AT LEAST THE MAJOR PORTION OF SAIDSOLID INSULATION, AND AN INTERMEDIATE SHIELDING MEMBER HAVING LOWER ANDUPPER PORTIONS FORMED FROM CONDUCTING MATERIAL DISPOSED TO SURROUND SAIDHIGH VOLTAGE WINDING AND AT LEAST A PORTION OF SAID LEAD, RESPECTIVELY,SAID LOWER PORTION OF SAID INTERMEDIATE SHIELDING MEMBER BEING EMBEDDEDIN SAID SOLID INSULATION SUBSTANTIALLY HALFWAY BETWEEN SAID FIRST ANDSECOND SHIELDING MEMBERS AND INTERPOSING AND DIVIDING SAID HIGH VOLTAGEWINDING INTO TWO SECTIONS, SAID INTERMEDIATE SHIELDING MEMBER BEINGELECTRICALLY CONNECTED TO SAID HIGH VOLTAGE WINDING AT A POINTINTERMEDIATE THE START AND FINISH OF SAID HIGH VOLTAGE WINDING, WITH THEPOINT ON SAID HIGH VOLTAGE WINDING BEING SELECTED TO OBTAIN A POTENTIALWHICH PROVIDES THE MAXIMUM REDUCTION IN POTENTIAL GRADIENT AT SAID FIRSTSHIELDING MEMBER, SAID UPPER PORTION OF SAID INTERMEDIATE SHIELDINGMEMBER BEING POSITIONED CLOSER TO SAID FIRST SHIELDING MEMBER THAN TOSAID SECOND SHIELDING MEMBER TO PRODUCE A CAPACITANCE RELATIONSHIPBETWEEN SAID SHIELDING MEMBERS THAT WOULD FORCE SAID INTERMEDIATESHIELDING MEMBER TO ASSUME THE SAME POTENTIAL THAT SAID INTERMEDIATESHIELDING MEMBER HAS ASSUMED DUE TO ITS BEING CONNECTED TO THE SELECTEDPOINT OF THE HIGH VOLTAGE WINDING.